Abstract

We conducted experiments to examine the effect of amphibole-forming hydration reactions on mineral fabric development and the strength of mafic rocks. Both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions (800 °C, 1 GPa). Amphibole that formed under hydrostatic conditions exhibits a random lattice-preferred orientation (LPO). In contrast, amphibole formed during deformation exhibits both a strong shape-preferred orientation (SPO) and LPO with a [001] maximum aligned subparallel to the shear direction. Plagioclase in both hydrostatic and deformed samples shows a very weak to random LPO. At low effective strain rates (10−5 s−1 to 10−6 s−1), the stress exponent is ≈1–1.5, consistent with deformation accommodated by diffusion creep. The correlation of the SPO and LPO coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation. The experimentally produced fabrics are strikingly similar to those observed in amphibolite-grade natural shear zones, supporting interpretations that such rocks deform by diffusion creep. In addition, the rheology of the fine-grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite. Thus, both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust.